The initial objective of the proposed research is the structural and functional characterization of skeletal muscle that is adapting to physiological hypertrophy. An experimental model has been developed utilizing a volitional effort to induce neuromuscular adaptation. This program consists of operantly conditioning cats to flex their wrist against increasing resistance. Moreover, this procedure has the advantage of inducing significant hypertrophy in only a few muscles of one limb, while the opposite limb can be utilized for comparative studies. Muscle histochemistry is used to classify the fiber types present in the forearm flexor muscles and to explore the patterns of adaptive reorganization that occur during physiological hypertrophy. After establishment of the characteristic structural alterations during the adaptive process, and an analysis of their temporal sequence, attention will be directed towards correlating structural data with functional studies. This phase of the study will involve physiological investigations of the wrist flexor muscles. Preliminary observations have shown striking adaptive changes in the morphological and histochemical profiles of exercised muscle fibers. Moreover, there is an indication that hypertrophy is the result of both muscle fiber hypertrophy and hyperplasia.